Due to the scarcity of spectrum resources and the explosion of heavy traffic services from mobile users, in order to increase user throughput and enhance mobility, there is an increasingly demand for hotspot coverage at high-frequency points (such as 3.5 GHz), and low-power nodes serve as new application scenarios. However, since signal attenuation at the high-frequency points is rather severe, new cells have relatively small coverage areas and do not share a station (STA) with the existing cells. Therefore, if the user moves between the new cells or moves between a new cell and an existing cell, it is inevitable to cause a frequent handover process, which leads to the frequent transfer of user information between base stations, thus resulting in a great signaling impact on a core network. To address this problem, a LTE network introduces a new system architecture, that is, a femtocell system. As shown in FIG. 1, the system architecture includes a mobility management entity (MME) 1, a serving gateway (SGW) 2, user equipment (UE) or referred to as a terminal 3, and base stations (eNBs) 4 including a master eNB 41 and a secondary eNB (SeNB) 42. There is a Uu interface between the UE 3 and each of the eNBs 4. There is a S1-MME (S1 for the control plane) interface between the MeNB 41 and the MME 1. There are S1-U interfaces between the MeNB 41 and the SGW 2 and between the SeNB 42 and the SGW 2. There is an X2 interface between the MeNB 41 and the SeNB 42. User data may be delivered from the core network to the UE through the MeNB or the SeNB. After the UE accesses the MeNB, dual connectivity may be achieved by adding, modifying, or deleting the SeNB.
Meanwhile, with the development of wireless multimedia services, the demand for high data rate and better user experience is growing, putting higher requirements on system capacity and coverage of traditional cellular networks. On the other hand, the popularity of applications such as social networks, short-range data sharing and local advertising increases the demand for proximity services, i.e., knowing and communicating with people or things of interest in the neighborhood. Traditional cell-based cellular networks have obvious limitations in high data rate and support of proximity services. Against this demand background, a device-to-device (D2D) technology on behalf of a new direction for development of future communication technologies comes into being. The application of the D2D technology may reduce the burden on the cellular networks, reduce the battery power consumption of the user equipment, increase the data rate, and improve the robustness of a network infrastructure, so as to meet requirements of the above high data rate service and the proximity service well.
D2D communications may reuse cellular communication resources. Against a scenario with cellular network coverage, the D2D communication resources are usually scheduled and allocated by the base station, in this way, resource reuse efficiency may be increased while effects of the control of D2D communication by a network side and the interference coordination between the D2D communication and cellular communication are ensured. For a same UE, if it supports a D2D function, the D2D communication with another D2D UE and the cellular communication with the base station may be performed at the same time.
When the UE is in the coverage of densely deployed small cells, there is no reasonable method in the related art to make full use of advantages of dual connectivity and the D2D technology to ensure the implementation of a D2D discovery/communication service uncer dual connectivity, (i.e., the D2D discovery or communication continues while the dual connectivity is realized).